U.S. patent number 9,352,416 [Application Number 13/377,597] was granted by the patent office on 2016-05-31 for niobium based superconducting radio frequency(scrf) cavities comprising niobium components joined by laser welding, method and apparatus for manufacturing such cavities.
This patent grant is currently assigned to THE SECRETARY, DEPARTMENT OF ATOMIC ENERGY, GOVT. OF INDIA. The grantee listed for this patent is Purushottam Das Gupta, Prashant Khare, Pradeep Kumar Kush, Chandrakant Pithawa, Sindhunil Barman Roy, Vinod Chandra Sahni, Brahma Nand Upadhyay. Invention is credited to Purushottam Das Gupta, Prashant Khare, Pradeep Kumar Kush, Chandrakant Pithawa, Sindhunil Barman Roy, Vinod Chandra Sahni, Brahma Nand Upadhyay.
United States Patent |
9,352,416 |
Khare , et al. |
May 31, 2016 |
Niobium based superconducting radio frequency(SCRF) cavities
comprising niobium components joined by laser welding, method and
apparatus for manufacturing such cavities
Abstract
Niobium or its alloy based Superconducting Radio Frequency
(SCRF) Cavities involving atleast one laser beam welded components
in the SCRF cavity welded from inside surface of the wall of cavity
directed to achieving more than half the thickness to full depth
penetration with minimum HAZ, minimizing distortion and shrinkage.
The method ensures improved weld quality and surface finish
substantially free of any weld defects. Also disclosed is the
welding nozzle system and welding rigs adapted to facilitate such
laser welding of the Niobium or its alloy based Superconducting
Radio Frequency (SCRF) Cavities. The invention is thus directed to
enhancing productivity, ensuring consistent quality and
reliability, enhanced weld penetration with minimum HAZ, smooth
finish of weld joints at possible reduced costs.
Inventors: |
Khare; Prashant (Indore,
IN), Upadhyay; Brahma Nand (Indore, IN),
Roy; Sindhunil Barman (Indore, IN), Pithawa;
Chandrakant (Trombay, IN), Sahni; Vinod Chandra
(Mumbai, IN), Gupta; Purushottam Das (Indore,
IN), Kush; Pradeep Kumar (Indore, IN) |
Applicant: |
Name |
City |
State |
Country |
Type |
Khare; Prashant
Upadhyay; Brahma Nand
Roy; Sindhunil Barman
Pithawa; Chandrakant
Sahni; Vinod Chandra
Gupta; Purushottam Das
Kush; Pradeep Kumar |
Indore
Indore
Indore
Trombay
Mumbai
Indore
Indore |
N/A
N/A
N/A
N/A
N/A
N/A
N/A |
IN
IN
IN
IN
IN
IN
IN |
|
|
Assignee: |
THE SECRETARY, DEPARTMENT OF ATOMIC
ENERGY, GOVT. OF INDIA (Maharastra, IN)
|
Family
ID: |
42309481 |
Appl.
No.: |
13/377,597 |
Filed: |
November 3, 2009 |
PCT
Filed: |
November 03, 2009 |
PCT No.: |
PCT/IN2009/000621 |
371(c)(1),(2),(4) Date: |
December 12, 2011 |
PCT
Pub. No.: |
WO2011/055373 |
PCT
Pub. Date: |
May 12, 2011 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20120094839 A1 |
Apr 19, 2012 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
39/2406 (20130101); B23K 37/0435 (20130101); H01L
39/14 (20130101); B23K 26/282 (20151001); B23K
26/106 (20130101); B23K 37/0533 (20130101); B23K
26/12 (20130101); H05H 7/20 (20130101); B23K
26/702 (20151001); B23K 26/147 (20130101); B23K
26/127 (20130101); B23K 26/0823 (20130101); B23K
26/32 (20130101); B23K 26/1224 (20151001); B23K
2103/08 (20180801) |
Current International
Class: |
H01L
39/12 (20060101); H01L 39/24 (20060101); B23K
26/00 (20140101); B23K 26/08 (20140101); B23K
26/10 (20060101); B23K 26/12 (20140101); B23K
26/14 (20140101); B23K 26/32 (20140101); B23K
37/04 (20060101); B23K 37/053 (20060101); H01L
39/14 (20060101); H05H 7/20 (20060101) |
Field of
Search: |
;219/121.6,121.63,121.64
;505/210,480,801,805,806 ;333/227 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Stapleton; Eric
Attorney, Agent or Firm: The PL Law Group, PLLC
Claims
We claim:
1. A method of producing Niobium based superconducting radio
frequency (SCRF) cavities having wall thickness over 1 mm to up to
3 mm and including laser welded component joints in RF field region
of said cavity obtained of Niobium sheets/components having
thickness over 1 mm to up to 3 mm comprising: carrying out pulsed
laser weld of said Nb sheet/component joints from inside in the RF
field region of said SCRF cavity achieving a depth of weld
penetration of more than 1 mm to full depth of up to 3 mm of the
said thickness of said Nb sheet/components welded following the
steps of: providing the said Nb sheet/components for said pulsed
laser weld joining with the butting edges thereof for welding in a
vacuum vessel in inert atmosphere and carrying out two step laser
welding of said butting edges of said components following: a first
phase of controlled key hole welding targeted at achieving said
depth of weld penetration comprising (a) carrying out keyhole
welding from inside surface of cavity wall of RF field region
formed of said butting edges using a single laser beam with varying
the temporal profile of pulsed laser including an initial
preheating of surface for desired penetration and tapering off the
energy near the end such as to achieve said desired depth of weld
penetration of welding from inside thereby minimising the heat
affected zone defects and (b) sucking out evaporated plasma
materials from laser and said Nb sheet/component metal at weld
joint; followed by a second phase of conduction welding targeted
only at achieving a smooth surface finish of weld joint enabling
desired SCRF activity comprising (a) carrying out conduction
welding also from inside surface of said keyhole weld joint
obtained in said first phase using two inclined laser beams for
weld joint smooth surface finish and also facilitating weld plume
rise perpendicular to the weld joint surface and (b) sucking out
said weld plume using suction arrangement to thereby provide for
said laser weld joint of said Nb sheet/components in the RF field
region from inside enabling SCRF cavities with laser weld joints
having wall thickness over 1 mm to up to 3 mm to perform desired
SCRF activity.
2. A method as claimed in claim 1 wherein said variation in the
temporal profile of pulsed laser comprises carrying out keyhole
welding comprised of depositing major energy in keyhole welding
phase comprising: i) providing only requisite amount of energy and
controlling the rate of heating by keeping pulse frequency low, and
ii) depositing energy with variation in time domain.
3. A method as claimed in claim 1 comprising carrying out laser
welding of the iris joint of SCRF cavity from inside surface of the
wall of the iris joint components between two half cells to form
dumbbells of SCRF cavity comprising: providing the half-cells held
together in an iris welding rig comprising a vacuum vessel;
carrying out in the first phase (i) keyhole welding from inside
surface of the wall of RF field region involving a nozzle having
three concentric cylindrical tubes, the laser beam passing through
the innermost tube, the outermost and innermost tube supplying gas
at high velocity and the middle tube connected to suction port,
selecting the welding parameters such that pulse profile is varied
in time phase to obtain full depth penetration with minimum
distortion and minimum heat affected zone; followed by (ii) in the
second phase carrying out the conduction welding to smoothen the
weld surface obtained in the first phase, involving inclined
nozzles brought at the level of joint wherein the beams are
inclined and the weld plume rises perpendicular to the surface, the
evaporated material formed because of laser material interaction
being sucked out through a material suction arrangement in form of
an enclosure.
4. A method as claimed in claim 1 comprising carrying out laser
welding of equator joint of single cell cavity or dumbbells
constituting the multi-cell SCRF cavity from inside surface of the
wall of cavity comprising: providing the dumbbells in insert and
carrying the same inside an equator welding rig comprising a vacuum
vessel with devices and/or attachments for carrying out the welding
from inside surface of the wall of the cavity; welding the equator
joints one after the other performing a two step welding operation
comprising: (i) in the first phase carrying out keyhole welding
with a laser beam inclined at an angle to the vertical in the plane
of welding in the range of 15.degree. to 45.degree., the energy
being varied in time phase for desired penetration and followed by
(ii) in the second phase carrying out conduction welding involving
said dual beam with beam inclination from the vertical in the range
of 30.degree. to 60.degree. in order to obtain a very smooth finish
after keyhole-welding operation is over, the evaporated material
from laser material interaction rising up perpendicular to the
surface of the joint and being collected by a suction arrangement
in the form of an enclosure.
5. A method as claimed in claim 4 wherein the welding methodology
comprises of the sequence, viewing butting edges--keyhole
welding--viewing--repairing--viewing--dual beam conduction
welding/smoothening--viewing the entire region with
boroscope--laser cleaning--final smoothening with defocused
laser--final inspection with boroscope.
6. A method as claimed in claim 1 wherein said keyhole welding is
carried out involving a welding nozzle comprising three concentric
tube type enclosure adapted to move up and down, the outermost and
inner most tube enclosure adapted to provide high velocity inert
gas to flow out whereas the middle enclosure is adapted to suck out
the evaporated material due to laser material interaction and
wherein said conduction welding is carried out involving the dual
beam whereby the evaporated material as a result of the laser
material interaction rises up perpendicular to the surface of the
joint and is collected by a separate enclosure adapted to move up
and down.
7. A method as claimed in claim 1 wherein the end group component
joints, which are in RF field free region, are welded from outside
surface of the wall of cavity involving keyhole welding only and
the ones, which are in RF field region, are welded by a combination
of keyhole and conduction welding from inside surface of the wall
of cavity.
8. A method of producing Niobium based superconducting radio
frequency (SCRF) cavities as claimed in claim 1 wherein Nd:YAG or
any other solid state laser is used for laser welding.
9. A method of producing Niobium based super conducting radio
frequency (SCRF) cavities as claimed in claim 1 comprising carrying
out of said laser welding for maximizing depth of penetration and
minimizing heat affected zone (HAZ), involving temporal variation
in energy within the pulse and selective repetition rate of the
said laser pulse.
10. A method of producing Niobium based super conducting radio
frequency (SCRF) cavities as claimed in claim 1 wherein temporal
profile of the pulse is tailored for Niobium or its alloys
preferably by varying the energy within the pulse in time domain in
a near trapezoidal shape such that initial preheating of surface
results in higher penetration and tapering off the energy near the
end is done in such a way to avoid micro cracks/porosity/other
defects, whereby the overall pulse shape ensures high penetration
depth and low heat affected zone.
11. A method as claimed in claim 1 wherein the laser beam is
incident at an angle to the surface such that the major evaporated
plasma materials from laser-material interactions is ejected along
a small solid angle and is sucked out through evacuation
nozzle.
12. A method as claimed in claim 1 comprising carrying out the said
keyhole welding and conduction welding involving inclined laser
beam with selective beam inclination for achieving desired
penetration with good surface finish wherein for said keyhole
welding the angle of inclination of the beam from the vertical is
in the range of 15 to 45.degree. preferably 30.degree. to vertical
and for conduction welding the beam inclination from the vertical
is in the range of 30 to 60.degree. preferably 45.degree. to
vertical.
13. A method as claimed in claim 1 comprising carrying out the
keyhole welding joints comprising welding by a laser beam striking
at an inclination/normal to the surface being welded by a welding
nozzle with three concentric openings wherein gas is purged from
the innermost and outermost openings while evaporated materials
from laser-material interaction is sucked out by the middle opening
adapted as an evacuation chamber.
14. A method as claimed in claim 1 comprising the step of
smoothening of surface of joints involving low intensity defocused
pulse laser beam adapted to penetrate only up to the modulations on
the surface.
15. A method as claimed in claim 1 comprising cleaning the weld
region and surroundings involving short pulses of nano-second
duration preferably laser pulses of low intensity preferably in the
region of 1-100 nano seconds and energy density in the range of
about 1-5 J/cm.sup.2 and involving methodology to also facilitate
sucking out of evaporating material due to laser material
interaction.
16. A method as claimed in claim 1 wherein the wall thickness of
the material being welded is more than 1 mm and the welding is done
by means of laser welding from inside surface or outside surface of
the wall of cavity having depth of welding ranging from greater
than half the thickness to full depth of the material being welded.
Description
CROSS REFERENCE TO RELATED APPLICATION
The present application claims all benefits accruing under 35
U.S.C. .sctn.365(c) from the PCT International Application
PCT/IN2009/000621, with an International Filing Date of Nov. 3,
2009, which claims the priority the entire contents of which are
incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates to superconducting Radio Frequency
(SCRF) Cavity and, in particular, to the fabrication of
superconducting Radio Frequency (SCRF) Cavity involving laser
welding with possible at least one welded joint welded from inside
surface of the wall of the cavity to a depth of even more than half
the thickness to full depth of the components welded with good
strength and improved weld characteristics of the joints. More
particularly, the invention is directed to Niobium and/or kits
alloy based superconducting Radio Frequency (SCRF) Cavity
fabrication involving possible laser weld from inside surface of
the wall of cavity to more than half the thickness to full
penetration for joints in the RF field region or in RF field free
region as the case may be, which is directed to favor obtaining
such weld joints much simpler with easy handling and manufacturing
steps. Importantly, the invention is directed to selective pulsed
laser beam welding of the Niobium and its alloy based components
for the fabrication of SCRF cavity with selective combinations of
keyhole and conduction welding to achieve large penetration depth
with minimum heat affected zone (HAZ), minimizing distortion and
shrinkage, reduced weld time and costs by way of
mechanized/automated weld execution with more controlled precision
and weld characteristics to benefit end uses as SCRF cavity with
improved weld quality and surface finish, particularly surface free
of weld defects/cracks that is highly detrimental for the SCRF
cavity. The invention is further directed to enable bulk
fabrication of SCRF cavities by laser welding of Niobium components
in a system with enhanced productivity, ensuring consistent quality
and reliability, enhanced weld penetration with minimum HAZ, smooth
finish of weld joints, in weld rigs adapted to suit such high
performance laser welding with enhanced productivity, quality and
reliability at reduced cost.
BACKGROUND ART
It is well known in the related field that the superconducting
radio frequency (SCRF) cavities are used to accelerate particle
beams in high-energy particle accelerators. These are in operation
for last 30-35 years. SCRF cavities have the advantage of operating
at high gradients, negligible AC power demand and favorable beam
dynamics. The present day technology for manufacturing of these
SCRF cavities is highly complex and expensive. Most of the SCRF
cavities are presently made from bulk niobium material involving a
fabrication process, which is exorbitantly costly and complex.
It is also known in the art that during the manufacturing of such
cavities, niobium components are joined by electron beam welding
(EBW) process. However, the existing electron beam welding (EBW)
process is applied to weld both types of joints of SCRF cavities:
those which are exposed to RF field and those that fall outside RF
field region; these have been termed as joints in RF field region
and joints in R.F. field free region respectively in the
embodiments. Conventional EBW process used for fabrication of the
SCRF cavities suffers from following drawbacks: a) It is an
expensive process as it has to be performed in vacuum and the
Capital cost of the EBW machine is very high. b) Chemical etching
required before every electron beam welding operation is an
expensive and hazardous operation, which, needs to be minimized, if
not eliminated. c) In order to weld intricate joints, complicated
manipulation of electron beam or parts to be welded is required.
Hence intricate joints are difficult to make with this process.
Joints made in SCRF cavities with EB welding, cause significant
amount of mechanical distortion and shrinkage.
It has also been experienced in the art that to weld Nb components
for SCRF cavity having high thickness (1 mm or above), the energy
deposited per unit time is high in case of EBW and as a consequence
the possibility of distortion or shrinkage is high. Moreover, weld
joints located in the RF field region of the SCRF cavity require
extremely smooth surface finish, since presence of any sharp points
on weld bead can cause field emission. This is difficult to achieve
with existing high energy EBW weld process consistently, which
causes detrimental effects on performance of SCRF cavities so
fabricated. The existing EBW weld process does not also provide
on-line, means for inspection of weld fit up, progress and finished
weld bead. The electron beam welding process as being applied today
makes it necessary that weld operations are carried out in many
settings and hence vacuum has to be broken many times which means
more time and cost. Also the existing weld systems and methods
applied in the SCRF cavity fabrication do not suggest any method
for removal of evaporating plasma material due to electron beam
material interaction from weld surface and a method for smoothening
for the intricate weld joints in precise weld set-up for the SCRF
cavity. The existing EBW welding process applied for fabrication of
the Niobium components of the SCRF cavity is thus expensive, low in
productivity and consistency in weld quality for Nb components.
U.S. Pat. No. 5,239,157 discloses a superconducting accelerating
tube which is constructed in a manner such that a plurality of
components, formed of a superconducting material and individually
having peripheral end portions adapted to be butted to one another,
are butted to one another at the peripheral end portions, and the
butting portions butted to one another are welded together. In the
superconducting accelerating tube according to the said '157, the
butting portions are welded by means of a laser beam, and the laser
beam is applied to the butting portions such that the components
are laser-welded to one another. Preferably, the accelerating tube
is designed so that at least only the respective inner surfaces of
the butting portions are laser-welded, and the depth of welding is
not greater than half the thickness of the superconducting material
and not smaller than 150 .mu.m.
Importantly the said U.S. Pat. No. '157 suggested that it was
preferred that the weld thickness should not be more than half the
thickness of the sheet. Particularly, it was stated in the said
prior art that if the depth of welding was greater than half the
thickness of the superconducting material, the dimensional accuracy
of the manufactured superconducting accelerating tube was lowered
by a contraction of the weld portion. It was further mentioned
under the said prior art that if the thickness of the
superconducting material was smaller than 0.1 mm, the strength of
the resulting superconducting accelerating tube was too low and the
wall of the tube would be too thin for satisfactory laser welding
and if the thickness of the superconducting material was greater
than 1 mm on the other hand, the thermal conductivity was low and
inevitably therefore the cooling efficiency obtained during the use
of the superconducting accelerating tube was low.
Thus it would be clearly apparent from the above teachings flowing
from the US prior art that under said art the thickness of material
"t" for weld was restricted in the region of 0.1.ltoreq.t.ltoreq.1
mm and moreover the weld had to have a limitation of not exceeding
beyond half the said thickness. However, usually the cavity wall
thickness for the purposes of SCRF cavities have over the years
been in the range of about 2 mm and more and in the present day
elliptical type SCRF cavities are generally made of Nb with
thickness of typically 3 mm. Hence the said prior art on laser
welding could not be applied in the industry which is manufacturing
superconducting cavities.
It would also be clearly apparent from the above state-of-the-art
that while the present day requirement of Nb based SCRF cavity
fabrication requires a wall thickness of 2 mm and above, the laser
welding of less than half the depth and up to 1 mm wall thickness
proposed in the U.S. Pat. No. 5,239,157 was not sufficient to
provide for weld joints with desired strength and other
characteristics for Nb based SCRF cavities. Moreover, the laser
weld of SCRF cavities proposed in the U.S. Pat. No. '157 could
never have provided for proper weld joints involving the standard
thickness of Nb of 2 mm or more used for fabricating SCRF cavities.
In particular attempting to achieve greater depth of welding beyond
the less than half depth of even less than 1 mm thickness attempted
and proposed in the US involving higher power lasers would have
invariably resulted in serious failure of the weld joints in terms
of high HAZ, large weld distortion and shrinkage.
There has, therefore, been a continuing requirement in the related
field to advance the technology of manufacturing/fabricating SCRF
accelerator cavities involving laser welding in order to achieve
higher reliability and productivity and simultaneously bring down
the manufacturing time and costs, to make the process and
equipments affordable as well as ensuring reliable weld performance
for fabrication of SCRF cavities. Involving Niobium and its alloy
based components with possibility of achieving welding from inside
surface of the wall of cavity to more than half the thickness to
even full depth of thickness which was not possible here to before.
Moreover, for such weld penetration using laser beam it has been
desirable to meet some of the critical requirements like good
surface finish, strength and freedom from risks of field emission
so that consistent performance from the RF joints can be obtained
while in service for charged particle acceleration in these
cavities. The other requirement in the art over the years had been
of meeting high penetration with minimized HAZ to ensure reliable
weld quality with high surface finish free of presence of any sharp
points in weld surface to avoid field emission, which is not
attainable by the existing EBW process consistently. Above all, the
intricacies and weld finish coupled with strength, smoothness of
weld bead that is required in particular for the RF field region of
the SCRF cavity, needed to be met by proper selection of process
and equipments, designing of the weld process and optimization of
parameters thereof to achieve economy, attaining user-friendly
provisions for such laser welding of SCRF cavity joints have been
the challenging constraints to adopting laser weld technique for
such fabrication of Nb based SCRF cavities.
OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide for
Niobium based superconducting radio frequency (SCRF) cavities
comprising Niobium and/or its alloy based components joined by
laser welding in the RF field region involving required strength
and desired weld characteristics to suit reliable performance of
cavity for accelerating charged particles and desired
superconducting properties free of any field emission, especially
by way of providing at east one weld joint of at least over half
the thickness to full depth penetration of the material being
welded from inside surface of the wall of the cavity.
Another object of the present invention is to develop a system and
method for welding of niobium and its alloy based components cost
effectively and also reducing functional difficulties in
fabrication of such SCRF cavities with enhanced productivity and
consistent quality and reliability, involving laser welding adapted
to overcome the limitations of the known laser weld method as well
as the conventionally followed EBW process used for such
purposes.
A further object of the present invention is directed to developing
a system for laser welding of Nb sheet/components for fabrication
of SCRF cavity comprising full penetration weld joints in RF field
region in the cavity, by a technique involving selective
combination of keyhole and conduction welding to achieve desired
depth of welding and also advantage of smooth weld bead and ease in
collection of evaporated plasma material produced due to laser
material interaction, so that there remains minimum chance of
presence of any sharp point on weld surface causing undesired field
emission in SCRF cavity.
A further object of the present invention is directed to developing
a method for laser welding of Nb sheet/components for fabrication
of SCRF cavity comprising laser welding of any type of cavity viz.
quarter wave resonator (QWR), half wave resonator (HWR), Re-entrant
type, elliptical type and the like SCRF cavity.
Yet another object of the present invention is directed to the
provision of system of weld rigs which are configured preferably to
weld elliptical type cavities with the desired atleast one weld
joint of at least over half the thickness to full depth penetration
of the material being welded from inside surface of the wall of the
cavity.
A further object of the present invention is directed to developing
a system for laser welding of Nb sheet/components for fabrication
of SCRF cavity capable of achieving full penetration weld having
more than 1 mm thickness and up to full depth of thickness of Nb
components with narrow HAZ, minimum distortion and shrinkage while
achieving desired strength requirements of such cavity.
A further object of the present invention is directed to developing
a system for laser welding of Nb sheet/components for fabrication
of SCRF cavity and a method thereof comprising use of selective
weld parameters such that high penetration and minimum HAZ is
achieved, by selection of process and optimization of weld
parameters including designing preferred pulse configuration in
temporal and spatial domain and frequency of its repetition.
A further object of the present invention is directed to developing
a system for laser welding of Nb sheet/components for fabrication
of SCRF cavity and a method thereof comprising welding of joints in
the RF field region involving smooth weld surface from inside
surface of the wall of cavity e.g. the inside surface of the wall
of butt weld joints of iris and equator joints of SCRF cavity, and
involving provision for required weld of some joints from outside
surface of the wall in case of joints in the RF field free
region.
A further object of the present invention is directed to developing
a method and system for laser welding of Nb sheet/components for
fabrication of SCRF cavity for welding of joints from inside
surface of the wall of cavity especially for the joints in RF field
region and also in the RF field free region involving good strength
and leak tightness,
A still further object of the present invention is directed to
developing a system for laser welding of Nb sheet/components for
fabrication of SCRF cavity wherein the weld joints surface is
cleaned using selective low energy and high frequency pulse for
desired faster laser cleaning and removing the evaporated plasma
material due to laser material interaction from weld surface,
minimizing conventional chemical cleaning which is time taking,
hazardous and costly.
A still further object of the present invention is directed to
developing a system for laser welding of Nb sheet/components for
fabrication of SCRF cavity involving a selective nozzle
configuration adapted to carry out the task of laser welding as
well as providing a gas jet and selective suction mechanism to suck
out of evaporating plasma material due to laser material
interaction by means of a vacuum pump connected to the concentric
annular passage inside the nozzle tip or by separate enclosures in
case of joints in RF field or RF field free region as these are
being welded by nozzle/nozzles inclined to the vertical so as to
favor collection and disposal of the plumes/debris generated at
weld site.
A still further object of the present inventions directed to
developing a system for laser welding of Nb sheet/components for
fabrication of SCRF cavity involving a set of welding rigs
configured with mechanized means to function as manipulator to
favor carrying out precise welding in selective weld locations of
SCRF cavity fabrication, including means for pre and post weld
on-line inspection of joints from out side of cavity, facilitating
faster and bulk production for enhancing productivity, including
carrying out weld rectification as necessary.
Yet another object of the present invention is directed to
superconducting cavities involving niobium and its alloy based
joined components which would be adapted to mitigate problems of
weld distortion and weld shrinkage in obtaining of such full depth
penetration of laser welded joints from inside surface of the wall
of SCRF cavities and means for on-line monitoring/control of
distortion due to weld.
Another object of the present invention is directed to the
manufacture of superconducting cavities involving niobium and its
alloy based welded joined components which would enable joining of
intricate shaped parts in such cavities with simplicity and
cost-effectively.
A further object of the present invention is directed to a system
and a method for carrying out selective simple and cost-effective
welding of niobium components for fabrication of SCRF cavities,
which would involve less capital cost or operating costs and ensure
superior quality of SCRF cavities.
A still further object of the present invention is directed to a
system and method for selective welding of R.F. field region of the
niobium based components for fabrication of SCRF cavities wherein
said selective welding could be carried out under vacuum or in an
inert gas atmosphere.
A further object of the present invention is directed to a system
and method for selective welding of the niobium based components
for fabrication of SCRF cavities, which would enable obtaining
fabrication of intricate shapes with desired
accuracy/precision.
A still further object of the present invention is directed to a
system and method for carrying out a selective laser welding of
Niobium based components for the SCRF cavities specifically
suitable for RF field region, wherein the weld process would ensure
full thickness penetration, minimum weld distortion and shrinkage
due to welding, narrow HAZ, smooth weld surface free of field
emission and in turn favor ensuring required dimensional accuracy
as well as reliable performance of the cavity in accelerating
charged particles.
A still further object of the present invention is directed to a
system and method for laser welding of niobium components for
fabrication of SCRF cavity, wherein for carrying out the welding of
joints involve provision for boroscopic inspection/viewing of the
weld region from remote location for proper control on weld
parameters for achieving desired weld quality.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention the
same is directed to providing Niobium or it's alloys based
superconducting radio frequency (SCRF) cavities, comprising of at
least one component made of Niobium or it's alloys which are joined
by laser welding from inside surface of the wall of the cavity with
depth of penetration of welding ranging from greater than half the
thickness to full depth of the material being welded.
Another aspect of the present invention is directed to Niobium or
it's allow, based superconducting radio frequency (SCRF) cavities
comprising iris joints and equator joints in the RF field region
formed by laser welding from inside surface of the wall of the
cavity with depth of penetration of welding ranging from greater
than half the thickness to full depth of the materials being
welded.
A further aspect of the present invention is directed to said
Niobium or it's alloys based superconducting radio frequency (SCRF)
cavities wherein anyone or more of said equator and/or iris joints
comprise full depth laser welding from inside surface of the wall
of the cavity.
Importantly, in said Niobium or it's alloys based super conducting
radio frequency (SCRF) cavities wherein said thickness of the laser
weld joint from inside surface of the wall of the cavity is greater
than 1 mm.
A still further aspect of the present invention is directed to said
Niobium or it's alloys based super conducting radio frequency
(SCRF) cavities comprising single and/or multi cell SCRF cavity for
charged particle acceleration.
A further aspect of the present invention is directed to said
Niobium or it's alloys based super conducting radio frequency
(SCRF) cavities comprising elliptical shaped Niobium or it's alloys
based super conducting radio frequency (SCRF) cavities wherein said
laser weld joints comprise anyone or more of laser weld dumbbells
produced from half cells, end group components welded to end group
main body including flanges and higher order mode (HOM) couplers,
weld joints in the RF field free regions including stiffening rings
welded from outside and RF field region joints including Equator
joints and iris joints welded from inside surface of they wall of
the said cavity.
According to yet another aspect of the present invention is
directed to a method of producing Niobium based superconducting
radio frequency (SCRF) cavities comprising:
providing atleast one component made of Niobium or it's alloys
joined by laser welding from Inside surface of the wall of the
cavity with depth of welding ranging from greater than half the
thickness to full depth of the material being welded following the
steps of (i) a first phase of controlled keyhole welding from
inside surface of the wall of cavity followed by (ii) a second
phase of conduction welding, to achieve very smooth finish of the
weld joint minimizing distortion and shrinkage with narrow HAZ and
weld surface finish adapted for achieving reliable operation of
SCRF cavity.
In the above method wherein said step of carrying out keyhole
welding comprises depositing major energy in keyhole welding phase
comprising: i) providing only requisite amount of energy and
controlling the rate of heating by keeping pulse frequency low, and
ii) depositing energy with variation in time domain.
A still further aspect of the present invention is directed to said
method comprising carrying out laser welding of the iris from
Inside surface of the wall of cavity of the iris joint between two
half cells to form dumbbells of SCRF cavity comprising:
providing the half-cells held together in an iris welding rig
comprising a vacuum vessel;
carrying out in the first phase (i) keyhole welding from inside
surface of the wall of cavity in RF field region involving a nozzle
having three concentric cylindrical tubes the laser beam passing
through the innermost tube, the outermost tube supplying gas at
high velocity and the middle tube connected to suction port,
selecting the welding parameters such that pulse profile is varied
in time phase to get high penetration with low distortion; followed
by (ii) in the second phase carrying out the laser welding
involving inclined nozzles brought at the level of joint wherein
the beams are inclined and the weld plume rises perpendicular to
the surface,
the evaporated material formed because of laser material
interaction being sucked out through a material suction arrangement
in form of an enclosure.
A still further aspect of the present invention is directed to said
method comprising carrying out laser welding of the equator joint
of single cell cavity or dumbbells constituting the multi cell SCRF
cavity from inside surface of the wall of cavity comprising:
providing the dumbbelis in insert carrying the same inside an
equator welding rig comprising a vacuum vessel with devices and/or
attachments for carrying out the welding from inside surface of the
wall of the cavity;
welding the equator joints one after the other performing a two
step welding operation comprising:
(i) in the first phase carrying out keyhole welding with a laser
beam inclined at an angle to the vertical in the plane of welding,
the energy being varied in time phase for better penetration and
followed by (ii) in the second phase carrying out conduction
welding involving dual beam in order to obtain a very smooth finish
after keyhole-welding operation is over, the evaporated material
from laser material interaction rising up perpendicular to the
surface of the joint and being collected by an enclosure.
A further aspect of the present invention is directed to said
method wherein the welding methodology comprises of the sequence,
viewing butting edges--keyhole
welding--viewing--repairing--viewing--dual beam conduction
welding/smoothening--viewing the entire region with
boroscope--laser cleaning--final smoothening with defocused
laser--final inspection with boroscope. In the above method wherein
said keyhole welding is carried out involving a welding nozzle
comprising three concentric tube type enclosure adapted to move up
and down, the outermost end inner most tube enclosure adapted to
provide high velocity inert gas to flow out whereas the middle
enclosure is adapted to suck out the evaporated material due to
laser material interaction and wherein said conduction welding is
carried out involving the dual beam whereby the evaporated material
as a result of the laser material, interaction rises up
perpendicular to the surface of the joint and is collected by a
separate enclosure adapted to move up and down.
Also in said method wherein the joints of end group component,
which are in RF field free region are welded from outside
involving, keyhole welding only and the ones, which are in RF field
region are welded by a combination of conduction and keyhole
welding
According to yet another aspect of the present invention, said
method of producing Niobium based superconducting radio frequency
(SCRF) cavities wherein Nd:YAG or any other solid state laser is
used for laser welding.
A further aspect of the present invention is directed to said
method of producing Niobium based super conducting radio frequency
(SCRF) cavities comprising carrying out of said laser welding for
maximizing depth of penetration and minimizing distortion,
shrinkage and heat affected zone (HAZ), involving temporal
variation in energy within the pulse and selective repetition rate
of the said laser pulse.
A still further aspect of the present invention is directed to said
method of producing Niobium based super conducting radio frequency
(SCRF) cavities wherein the temporal profile of the pulse is
tailored for Niobium or its alloys preferably by varying the energy
within the pulse in time domain in a near trapezoidal shape such
that initial preheating of surface results in higher penetration
and the tapering off the energy near the end is done in such a way
to avoid micro cracks/porosity/other defects, whereby the overall
pulse shape ensures high penetration depth and low heat affected
zone.
A still further aspect of the present invention is directed to said
method wherein the laser beam is incident at an angle to the
surface such that the major evaporated plasma material from laser
material interactions ejected along a smaller solid angle only and
is sucked out through evacuation nozzle.
A still further aspect of the present invention is directed to said
method comprising carrying out the said keyhole welding and
conduction welding involving inclined laser beam with selective
beam inclination for achieving desired penetration with good
surface finish wherein for said keyhole welding the angle of
inclination of the beam from the vertical is in the range of
15.degree. to 45.degree. preferably 30.degree. to vertical and for
conduction welding the beam inclination from the vertical is in the
range of 30.degree. to 60.degree. preferably 45.degree. to
vertical.
In the above method of the present invention wherein carrying out
the keyhole welding joints comprising welding by a laser beam
striking inclined/normal to the surface being welded by a special
welding nozzle with three concentric openings wherein gas is purged
from the innermost and outermost openings while evaporated
materials from Laser-material interaction is sucked out by the
middle opening adapted as an evacuation chamber.
A still further aspect of the present invention is directed to said
method comprising cleaning the weld region and surroundings
involving short pulses of nano-second duration preferably laser
pulses of low energy preferably in the region of 1-100 nano seconds
and energy density in the range of about 1-5)/cm.sup.2. The
important aspect in this case being the sucking out of evaporating
material due to laser material interaction.
A further aspect of the present invention is directed to said
method comprising the step of smoothening of surface of joints
involving low intensity defocused pulse laser beam adapted to
penetrate only up to the modulations on the surface.
Importantly also in said method, wherein the wall thickness of the
material welded is more than 1 mm and the welding by laser welding
from inside surface of the wall of cavity having depth of
penetration of welding ranging from greater than half the thickness
and full depth of the material being welded.
According to yet another aspect of the present invention is
directed to a keyhole welding nozzle for use in laser welding of
Niobium based superconducting radio frequency (SCRF) cavities and
the like comprising:
three concentric tubular members defining a nozzle head at the
front and a hollow cylindrical shaft in the center accommodating a
lens assembly for welding and viewing of the weld region;
three concentric tubes like enclosure assembly having a central
region enclosure accommodating a lens assembly and necessary optics
to focus the laser beam for welding and for viewing the welding
zone along with a provision for high velocity gas for purging;
the second enclosure after the central region is adapted to be wide
enough for sucking out the evaporated material due to laser
material interaction and the outermost third enclosure is adapted
to purge high velocity gas to minimize spread of evaporated
material;
the outermost and innermost tube enclosure of nozzle head provided
for high velocity inert gas to flow out whereas the middle
enclosure is adapted for sucking out the evaporated material due to
laser material interaction.
A further aspect of the present invention is directed to a
conduction welding nozzle system for use in laser welding of
Niobium based superconducting radio frequency (SCRF) cavities and
the like comprising:
nozzles meant for delivery of dual beam laser with lens assembly
adapted such that the beams are inclined preferably about
45.degree. with respect to the vertical and having means for
sucking out the weld plume rising perpendicular to the surface
through an enclosure adapted to be moved up and down
telescopically.
A further aspect of the present invention is directed to a welding
rig which is the type I welding rig such as for carrying out
welding of elliptical type SC cavities and the like comprising:
a vacuum vessel with motor driven attachments for holding and
manipulating the half cells, aligned to form dumbbells of SCRF
cavity for welding of cavity joints;
nozzle alignment mechanism;
keyhole nozzle means and conduction welding nozzle system
selectively disposed and adapted to sequentially carry out the
keyhole and conduction welding from inside surface of the wall of
the dumbbell within said cylindrical vacuum chamber.
The above welding rig comprising laser welding nozzle selectively
disposed for carrying out said laser welding of cavity joints from
inside surface of the wall of cavity for joints in RF field region
and from outside the surface for joints in RF field free
regions.
Also in said welding rig wherein said welding nozzles are adapted
to carry out inclined welding along the line of the weld joint but
in the plane of periphery to be welded in the first phase by
keyhole welding and followed by second phase conduction welding
from inside surface of the wall of the cavity.
Further the above welding rig wherein said welding nozzles are
adapted to carry out key hole welding from outside surface for
joints in the RF field free region for stiffening rings and some
end group components.
A still further aspect of the present invention is directed to a
welding rig which is the type II welding rig such as for carrying
out welding of elliptical type SCRF cavities and the like
comprising:
a vacuum vessel with different devices/attachments, to carry out
laser welding of joints of Niobium components of SCRF cavity,
specifically for the equator joints of SCRF cavity adapted to house
a rig insert adapted to assemble together welded half cells forming
the dumbbells;
encoders and means for precise positioning of the nozzles from
outside;
means preferably boroscopes provided to inspect the surrounding
regions after welding is over.
said rig insert comprising two circular flanges held together by
three tie rods spaced at 120.degree. adapted to assemble together
welded half cells forming the dumbbells and facilitate laser
welding such as of equator joints of Niobium components of SCRF
cavity;
strain gauges assembled in this insert adapted to monitor the
distortions online during welding.
The above welding rig wherein
said different devices/attachments comprises of a vacuum vessel, a
tie rod mechanism to hold dumbbells system, provision for
evacuation of vessel and purging with inert gas and seals for the
different protrusions in the vacuum environment;
said means for precise positioning of the nozzles from outside
comprises of a hollow cylindrical shaft capable of rotating about
it's own axis carrying laser heads and also enclosures which move
up and down to collect evaporated material from laser material
interaction, said rig also including a mechanism involving two
motors, ball screws and encoders which remain outside vacuum
environment and provide precision rotary and axial movement to
nozzles which are read by the encoders, operatively connected
pipelines in the rig adapted to take out evaporated material from
the welding region to outside the vacuum vessel.
A still further aspect of the present invention is directed to a
rig system for carrying out welding of elliptical type
superconducting radio frequency cavity joints both in the RF field
region and RF field free regions comprising:
a set of atleast two welding rigs comprising (i) a welding rig
assembly adapted for welding of half cells to form dumbbells and
end group components; (ii) a welding rig assembly adapted for
carrying out welding of equator joints from inside surface of the
wall of the SCRF cavity.
A further aspect of the present invention is directed to a system
for carrying out welding of elliptical type SCRF cavity components
wherein each of the said rigs comprise weld accessories including a
nozzle assembly with optical fiber connection adapted to facilitate
delivery of laser beam in the form of controlled pulses which is
adapted to deliver high energy pulse for keyhole welding, means for
low energy laser pulses for conduction welding, defocused low
energy pulses for weld smoothening and cleaning by nano second
duration laser pulses, means for viewing of butting edges and
welding of joints intermittently, means to inspect defective region
of cavity and rectify regions having problems, means for weld
smoothening and cleaning with removal of evaporated plasma
materials from laser material interaction and means for evacuating
the vessel and purging it by helium or any other suitable inert
gas.
A still further aspect of the present invention is directed to said
system wherein in said welding rig assembly the optical fiber is
provided to carry four types of pulses comprising (a) high energy
pulses having variation in time domain for carrying out keyhole
welding in the RF field region as well as RF field free region (b)
low energy pulses for conduction welding (c) pulses of lower energy
adapted to function as defocused beam and (d) applying low energy
pulses of nano second duration to those areas of the cavity where
some surface defects are seen for laser cleaning.
The present invention, its objects and advantages are described in
greater details with reference to the accompanying non-limiting
illustrative Figures.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
FIG. 1: is the schematic illustration of the Type I welding rig for
carrying out laser welding of SCRF cavities in accordance with the
present invention;
FIG. 1A: is an enlarged illustration of the arrangement of half
cells forming the dumbbells housed in the Type I rig of FIG. 1 and
a section AA of the same enlarged view in the dumbbell;
FIG. 1B: is the schematic illustration of the Type I welding rig
for carrying out laser welding of stiffening ring on the dumbbell
of SCRF cavities in accordance with the present invention;
FIG. 2: is the schematic illustration of the cross sectional view
of an embodiment of the welding nozzle Type A in accordance with
the invention;
FIG. 3: is the schematic illustration of the process of dual laser
beam nozzle Type B based welding with provision for separate
suction pipe for removal of evaporated plasma material
FIG. 4a: shows the main body of an end group of an SCRF cavity;
FIG. 4b: illustrate the other components such as the HOM coupler,
the NbTi ring and the coupler flange of an end group assembly.
FIG. 4c: illustrates the set up for welding of the NbTi ring on
flange inside Type I welding rig of the invention;
FIG. 4d: illustrates the set up for welding of HOM coupler in a
Type I welding rig of the invention which shows keyhole welding and
conduction welding/smoothening with only one beam through a nozzle
which can rotate about it's own axis;
FIG. 4e: illustrates the set up for welding of half-cell on main
body inside Type I welding rig of the invention;
FIG. 5: is the schematic illustration of the insert assembly
required to carry out the welding in a Type II welding rig in
accordance with the present invention;
FIG. 6: is the schematic illustration of the welding rig of Type II
along with insert carrying the dumbbells for, welding, placed
inside, according to the present invention;
FIG. 7: is the schematic illustration of Type II weld Rig without
the insert assembly to show the detailed arrangements used in said
welding rig involving all accessories/attachments/drives for laser
welding and carrying out other desired functions;
FIG. 8: is the schematic illustration of the enlarged sectional
view on B-B (as marked in FIG. 7) of the keyhole welding process
carried out using welding nozzle of Type A;
FIG. 9: is the schematic illustration of the enlarged sectional
view on A-A (as marked in FIG. 7) showing the manner of the
conduction welding operation, which is performed after keyhole
welding is over, using Type B welding nozzle;
FIG. 10: is the schematic illustration showing the set up for
on-line boroscopic examination/inspection of the weld zone in a
Type II welding rig;
FIG. 11: is the illustration of a typical near trapezoidal pulse
shape configuration used for carrying out the pulsed laser welding
in accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
Reference is first invited to accompanying FIG. 1 which is a
schematic illustration of the welding rig of Type 1 having a simple
cylindrical vacuum chamber with motor driven attachments for
holding the dumbbells to weld such as the iris joints to form
dumbbells and stiffening rings on the dumbbells of SCRF cavity.
As clearly illustrated in said figure, the Type I rig basically
involves the cylindrical vacuum chamber which accommodates a rotary
mechanism for supporting a pair of half cells forming the dumbbell
and adapted for rotary motion for facilitating the welding. For the
purpose of effecting the welding from inside surface of the wall of
the cavity in the RF field region, a nozzle is provided. Another
set of welding nozzles is also provided for welding other parts of
end group. All nozzles are provided with precise alignment
mechanisms.
Importantly, the Type I rig is adapted to carry out the welding
from inside surface of the wall of the half-cell within the RF
field region. There is a separate nozzle means (NA) to weld the
components of end group main body with end group components. For
the purpose of carrying out the welding from inside surface of the
wall of half cells in the RF field region such as the iris joint
with depth of penetration of welding ranging from greater than half
the thickness to full depth of the material being welded, the Type
I rig provides nozzles for keyhole welding and also conduction
welding. Such an arrangement of selective provision for both a
keyhole welding nozzle and conduction welding nozzles is shown in
the following FIG. 1. FIG. 1A: is an enlarged illustration of the
arrangement of half cells, forming the dumbbells housed in the Type
I rig of FIG. 1 and a section AA of the same enlarged view in the
dumbbell;
Advantageously, the provision of such nozzle and nozzle maneuvering
arrangements are provided such as to facilitate the laser welding
from inside surface of the wall of the half cell in the RF field
region involving selectively (i) in the first phase carrying out
keyhole welding with a laser beam inclined at an angle to the
vertical in the plane of welding, the energy being varied in time
phase for better penetration and followed by (ii) in the second
phase carrying out conduction welding involving dual beam in order
to obtain a very smooth finish after keyhole-welding operation is
over, the evaporated material from laser material interaction
rising up perpendicular to the surface of the joint and being
collected by an enclosure.
In keeping with another aspect of the invention the same thus
provides for two varieties of nozzles the type A which is used for
carrying out the keyhole welding and Type B which is used for
carrying the conduction welding with dual beam provision.
Reference is now invited to accompanying FIG. 2 which shows by way
of a schematic illustration of the cross sectional view of an
embodiment of the welding nozzle Type A used for welding joints by
keyhole welding technique following the present invention. Its
different elements are adapted to carry out the combined tasks of
laser welding, providing two gas jets and suction mechanism to
remove the evaporated material from the weld joint location. The
lens assembly remains stationary and the enclosures move up and
down.
Reference is now invited to accompanying FIG. 3, which shows Type B
welding nozzle used in the present invention. This operation is
performed in order to obtain a very smooth finish after
keyhole-welding operation is over. Dual beam welding technique is
used here. A major advantage here is that the evaporated material
as a result of laser material interaction rises up perpendicular to
the surface of the joint and is collected by an enclosure, which
can be moved up and down. The lens assembly gives a focal length
such that the two beams are focused on the butting edge.
FIG. 3 clearly illustrates the principle of dual beam welding with
provision for removal of evaporated material. It shows welding
nozzle of Type B, which aims to carry out laser welding of joints
of Niobium components of SCRF cavity, for joints, which are in the
RF field region. The welding is performed from inside surface of
the wall of cavity with conduction welding technique after keyhole
welding is over. The main feature is the enclosure for collecting
the evaporated material, which can move up and down telescopically.
A typical case of welding with the help of two beams, which make an
angle of 45.degree. with the vertical, has been illustrated.
Thus the arrangement of the said keyhole and conduction welding
nozzles are such as to favor providing inclined laser beam with
selective beam inclination for achieving desired penetration with
smooth bead wherein for said keyhole welding the angle of
inclination of the beam is in the range of 15.degree. to 45.degree.
preferably 30.degree. to vertical and for conduction welding the
dual beams are inclined to vertical in the range of 30.degree. to
60.degree. preferably 45.degree.. It is pertinent to point out that
even for keyhole welding operation an inclination is being given so
that the resulting weld bead, is smooth. Had the beam been incident
at 90.degree. to the surface although the penetration would have
been higher the bead would have been poor and then mending it in
the conduction welding phase would have been difficult.
Reference is now invited to accompanying FIG. 4a, which shows the
main body of an end group used in an SCRF cavity involving laser
welding in accordance with the present invention.
FIGS. 4c, 4d and 4e show the possible welding of NbTi ring on
flange, the welding of the HOM coupler and also the welding of the
half cell on main body all of which can be carried out using the
welding rig Type I shown in FIG. 1. Importantly, the same provision
for simple cylindrical vacuum chamber with motor driven attachments
for supporting cavity components on a rotary platform adapted for
rotary motion for facilitating the welding is shown. Welding nozzle
arrangement for effecting the welding from inside surface of the
wall of the end group component joints, which are in the RF field
region is provided. For welding end group component joints in RF
field free region, nozzle arrangement from outside is provided.
Reference is now invited to accompanying FIG. 5, which shows
schematic illustration, of the insert of welding rig of type II
according to the present invention. As shown in said figure this is
an insert comprising two circular flanges held together by three
tie rods spaced at about 120.degree., to carry out laser welding of
equator joints of Niobium components of SCRF cavity. The
half-cells, which were welded to make a dumbbell in type I welding
rig are precisely assembled in this insert. The precisely shaped
spacers are used for exact location. Strain gauges are also
assembled in this insert to monitor the distortions online during
welding. FIG. 6 is the schematic illustration of the welding rig of
type II along with insert carrying the dumbbells for welding.
According to the present invention this rig comprises of a vacuum
vessel with different devices/attachments, to carry out laser
welding of joints of Niobium components of SCRF cavity,
specifically for the equator joints of SCRF cavity.
The insert, illustrated in FIG. 5 is first installed in the chamber
and then welding takes place. The welding methodology comprises of
the sequence, viewing butting edges--keyhole
welding--viewing--repairing--viewing--dual beam conduction
welding/smoothening--viewing the entire region with
boroscope--laser cleaning--final smoothening with defocused
laser--final inspection with boroscope.
In this rig the welding operation is performed in two steps. In the
first step the keyhole welding process with the help of a beam,
which is inclined at an angle to the vertical in the plane of
welding is carried out. The energy is varied in time phase in this
case for better penetration. Here the lens assembly does not move
and it remains within the hollow cylindrical shaft. The three
concentric tube type enclosures move up and down. The outermost and
innermost tube enclosures are used for high velocity inert gas to
flow out whereas the middle enclosure is used for sucking out the
evaporated material due to laser material interaction.
In the second stage conduction welding operation is performed in
order to obtain a very smooth finish after keyhole-welding
operation is over. Dual beam welding technique is used here. A
major advantage here is that the evaporated material as a result of
laser material interaction rises up perpendicular to the surface of
the joint and is collected by an enclosure, which can be moved up
and down
FIG. 7 is an illustration of the mechanism, which will be used in
Type II welding rig. It illustrates some important features of this
rig. Importantly, the system is adapted to provide for operative
gadgets in a limited space available inside the cavity, for
different welding contraptions like nozzles and optical fibers. The
welding rig uses encoders, ball screws and other accessories so
that precise position of the nozzles can be known from outside. A
very significant aspect of this rig is the use of a ball screw and
a cylindrical pipe in conjunction with each other so that precision
of the ball screw can be utilized without the ball screw having to
be inserted in the limited space inside the cavity. Boroscopes are
provided to inspect the surrounding regions after welding is
over.
FIG. 8 is the view of the keyhole welding process, which is taking
place with the help of welding nozzle of Type A. Here the lens
assembly does not move and it remains within the hollow cylindrical
shaft. The three concentric tube type enclosures move up and down.
The outermost and innermost ring enclosures are used for high
velocity inert gas to flow out whereas the middle enclosure is used
for sucking out the evaporated material due to laser material
interaction. The nozzle is inclined in the plane of welding as is
evident from the figure.
FIG. 9 is the view of the conduction welding operation, which is
performed after keyhole welding is over. This process uses Type B
welding nozzle. This operation is performed in order to obtain a
very smooth finish after keyhole-welding operation is over. Dual
beam welding technique is used here. A major advantage here is that
the evaporated material as a result of laser material interaction
rises up perpendicular to the surface of the joint and is collected
by an enclosure, which can be moved up and down. The lens assembly
gives a focal length such that the two beams are focused on the
butting edge.
As discussed in relation to FIG. 3, the same principle of dual beam
welding with provision for removal of evaporated material is
followed. It involves welding nozzle of Type B, arranged in welding
rig of type II according to the present invention, which aims to
carry out laser welding of joints of Niobium components of SCRF
cavity, which are in the RF field region. The welding is performed
from inside surface of the wall of the cavity with conduction
welding technique after keyhole welding is over. The main feature
is the enclosure for collecting the evaporated material, which can
move up and down telescopically. A typical case of welding with the
help of two beams, which make an angle of 45.degree. with the
vertical has been illustrated.
FIG. 10 shows the provision for boroscpic examination of the
surrounding region of weld, which is also provided in the type II
weld rig of the invention.
FIG. 11: illustrates a typical pulse shape configuration (near
trapezoidal) according to the invention to carry out the pulsed
laser beam welding in the RF field region as well as RF field free
region by performing keyhole welding with varying the pulse energy
in time and space domain, adapted to achieve very good penetration
with minimum HAZ along with low distortion and shrinkage.
Importantly, it is thus possible employing the above selective
manner of manufacture of SCRF cavities to achieve for the first
time Niobium or it's alloys based superconducting radio frequency
(SCRF) cavities, comprising of at least one component made of
Niobium or it's alloys which are joined by laser welding from
inside surface of the wall of the cavity with depth of penetration
of welding ranging from greater than half the thickness to full
depth of the material being welded.
The full penetration weld (keyhole welding) could be obtained from
inside surface of the wall of cavity with the provision of the Type
A nozzle, which comprises three concentric tubes. The laser beam
travels through the innermost tube. The outermost ring supplies gas
at high velocity and the middle tube is connected to suction
device. The welding parameters are such that pulse profile is
varied in time phase to minimize energy deposited for same
penetration depth thereby reducing distortion. The welding nozzle
is inclined at an angle of 30-45.degree. from vertical. The
preferred angle is 30.degree. to get a high penetration depth with
good weld surface finish.
In the second step the inclined conduction weld nozzles are brought
at the joint. Here the beams are inclined at 45.degree. and the
weld plume rises perpendicular to the surface. The evaporated
material formed because of laser material interaction is sucked out
through a separate evacuation enclosure adapted to move up and down
in relation to the weld joint. Advantageously also following such
operation a much more smooth bead could be obtained in this
step.
It is important to note that in comparison to welding from two
sides involving joints in the RF field region, weld to achieve a
weld depth of more than half the thickness to up to full depth, the
possible welding to such depth from inside surface of the wall of
the RF field region achieved by the present invention following the
combination of keyhole welding followed by conduction welding for
the first time attains additional advantages as discussed
hereunder: A. Lesser number of weld settings, thereby saving time
and money. B. When welding in a single setting it is possible to
avoid the problems, which are encountered during handling. A half
welded cavity when removed from a rig and the fitted in the next
rig may deform or get contaminated while handling. C. Reinstalling
the cavity in rig means that there is a risk of setting it up at a
different position.
Lastly there always remains a risk whether the two fusion zones
have met or is there a region which is unwelded and in such case
whether it is conduction welding followed by keyhole welding or
otherwise, problem with weld quality is expected. If the conduction
weld is carried out firstly from inside surface of the wall of the
cavity and the keyhole weld after this from outside then it may be
that there is more than required penetration and spoiling of the
surface finish and if it is the other way round then the risk of
having an unwelded region is high as we don't get very good
penetration depth with conduction welding which is aimed at getting
a good surface finish.
Advantageously, it would be further apparent from the details of
the system and method of the invention that the same also provides
means for weld cleaning process by low energy pulsed laser of nano
second duration wherein the evaporated plasma materials from laser
material interaction are removed by sucking from weld zone using
vacuum pumps of appropriate suction head. A flexible nozzle with
multiple concentric circular passage section having optical fibre
at center with suitably designed optics for carrying laser pulse
for welding as well as purging inert gas at high velocity for
protection of weld pool and driving plumes in vertically upward
direction so that an integral or independent suction means deployed
to suck the evaporated plasma materials from laser base
material/metal interaction from weld zone. For keyhole welding with
single nozzle is either in vertical orientation or low inclination
with vertical and for conduction welding, dual nozzle with inclined
orientation to vertical is used to perform weld surface finishing
as well as favor suction of evaporated plasma materials from the
laser material interaction. Importantly, the boroscope means with
retraction facility favor carrying out pre and post-weld on-line
inspection for fit up and defect free weld joint providing facility
for optical inspection of the fit-up and weld finish/cleaning
operations.
The system according to the present invention is further directed
to carry out the laser welding of the SCRF cavity at different weld
locations of the cavity by using controlled pulse parameters of
pulsed laser welding such as the selective pulse shape in time
domain, pulse repetition, pulse duration and pulse overlap in space
domain depending on if the welding is in RF field and outside the
RF field region vis-a-vis the requirement of pulse energy input to
meet desired requirements of weld finish, reduced
distortion/warpage of cavity, defect free weld with smooth surface
finish for avoiding emission prone sharp points of weld joints and
strength needed during service for charged particle
acceleration.
Importantly also, to overcome the limitations in application of
laser welding of components made of Niobium or its alloys of
thickness of more than 1 mm or higher in the field of fabrication
of SCRF cavity especially from inside surface of the wall of the
cavity, preferably using any Solid State laser such as Nd:YAG
laser, Yb:YAG laser, Fiber laser, or any other laser that can be
delivered through optical fiber the present invention ensures
superior weld quality adequate for application in SC cavities by
incorporating new and inventive technical features comprising (a)
Adopting a combination of keyhole welding and conduction welding
for the joints in RF field region from inside surface of the wall
of the cavity and other selective weld locations to perform more
than half the thickness to full penetration weld joints for Niobium
sheets/components having thickness over 1 mm and typically up to 3
mm by using high energy pulsed laser welding with controlled energy
input to minimize distortion/shrinkage and confine to narrow HAZ,
using said weld system/rig, nozzle, sequence and selective pulse
characteristics/parameters; and conduction welding for bead
smoothening using dual laser beam with nozzles symmetrically
inclined to normal along plane of weld, carrying low energy pulsed
laser beams and merging simultaneously on weld joint; b)
Selectively designing pulse parameters comprising the pulse shaping
using preferably a near trapezoidal configuration having favorable
characteristics, pulse duration, frequency of pulse repetitions,
pulse overlapping to match the weld joint characteristic required
to comply with particular application; c) The possibility of
occurrences of surface roughness/weld ripples on bead surface is
smoothened by a defocused laser beam of millisecond duration and
laser cleaning of inner surface by low energy pulse of nano second
duration; removing the evaporated material due to laser material
interaction in both cases. d) To suck out evaporative plasma
materials from the laser material interaction, involving specially
designed nozzle having evacuation enclosure connected to vacuum
pump means to create desired volumetric capacity and vacuum head
required for such cleaning and thus minimizing possibilities of
surface defects that may lead to defects like field emission while
in service; e) Smooth weld bead is ensured for reliable performance
of the SCRF cavity minimizing the requirement of heavy chemical
processing minimizing associated problems of time taking, hazardous
and costly process. f) Using specially configured weld rigs and
contraptions for each type and location of weld joints in the
multicell SC cavities requiring meeting specific quality criteria
based on service/end application, adapted for faster/automated
process and equipments with reliable weld quality to suit industry
scale application in a simple and user friendly manner.
The system of laser welding for Niobium welding for fabrication of
SCRF cavities according to the present invention and the manner of
implementation of the laser welding from inside surface of the wall
of the cavity using the welding rigs of two categories e.g. Type I,
& II as discussed above for different weld locations in the
fabrication of the SCRF cavity are described in further details
with reference to specific embodiments and drawings in following
broad categories: 1) Welding of half cells to form dumbbells in
Type I rig. 2) Welding of end group components on to the main body
of end group in Type I rig. 3) Welding of Equator joints of SCRF
cavity from inside surface of the wall of cavity and welding of end
group with end half cell to the rest of cavity using the insert and
weld rig of Type II;
BRIEF DESCRIPTION OF THE PROCESS
The Following Steps Illustrate how an Elliptical Type SCRF Cavity
can be Fabricated Using this Present Invention
STEP-1: Welding Iris Joints from Inside Surface of the Wall of Half
Cell in the Type I Welding Rig to Form Dumbbell from Two Half
Cells
Reference is now invited to the accompanying FIG. 1 which is
schematic illustration of the laser beam welding of the pre-formed
half cells from Nb based sheet material welded together to form
dumbbells (DB) used to fabricate SCRF cavity using the Weld Rig of
Type I (WR-I) having a simple cylindrical vacuum chamber with
motorized attachments for holding (MH) and manipulating wherein
welding of joints in RF field and field free region and in
particular the set up for keyhole welding followed by dual laser
beam conduction welding of iris joint (IJ) from inside surface of
the wall of cavity has been shown. The accompanying FIG. 1 also
shows an enlarged view of the dumbbell (DB) portion wherein the `A`
type nozzle (NA) is used for keyhole welding as first operation
followed by dual laser beam for weld smoothening using conduction
welding in sequence/succession as second stage of operation, have
been shown. A sectional plan view on A-A of the dumbbell on the
plane of iris joint (IJ) shows the selective inclination to
vertical disposition of the dual nozzles of `B` type (NB). These
nozzles are having retractable mounting with respect to the rig to
move up down as and when necessary to maintain desired sequence of
weld operation and focusing with respect to a specified weld joint.
It has been experimentally found that the optimized weld parameters
include said keyhole welding carried out with the angle of
inclination of the beam from the vertical in the range of
15.degree. to 45.degree. preferably 30.degree. to vertical and for
conduction welding the beam inclination from the vertical is in the
range of 30.degree. to 60.degree., preferably 45.degree. to
vertical for desired bead smoothening to obtain joints free of
defects including avoiding sharp points on weld surface prone to
field emission while in service. The weld rig is provided with
nozzle alignment mechanism (NAM) with feed through (Fr) arrangement
enabling precisely focusing the laser beam on the joint to be
welded. The job is held by means of a motorized holder (MH) that
receives rotary motion to carryout the circumferential weld seam at
desired speed of rotation to match desired welding speed. At the
first stage of keyhole welding of iris joint in Type I rig, the `A`
type nozzle used is having inbuilt concentric cylindrical
passage/tubes (CR) for evacuation of the evaporating plasma
materials from the weld site. For type B nozzle an evaporated
plasma material suction device (SCD) is located perpendicular to
weld surface at central position between the two inclined laser
beams adapted to sucking in and disposing off the plumes that is
directed in central vertically upward direction by the two laser
beams at inclined orientation to vertical from two sides of the
welding site.
The vessel is now closed and evacuated to a vacuum of 10.sup.-5
mbar. Suitable ultra pure inert gas-either of helium, or argon is
then purged in this vessel to displace the air trapped inside.
After a couple of such cycles, it can be reasonably ensured that no
contaminating gas is present in the chamber.
The vacuum vessel (VV) is in complete inert atmosphere with the job
mounted on motorized positioner/holder inside the vessel. The joint
is then checked for proper fit up visually from both inside and
outside. The holding cum manipulating mechanism is used to ensure
that the beam focal point is perfectly matched with butting line of
two half-cells. As the next step a He--Ne laser is then used to
check the focal point of the impending beam.
The iris joint which is located in the RF field region, is welded
by keyhole welding technique using laser welding nozzle type `A`
(NA), in near vertical or slightly inclined to vertical
orientation. The joint is viewed with the help of a viewing
mechanism that uses the same lens mechanism, which was used to weld
the joint. If there is some region, which is unwelded, then it is
welded again without opening the chamber. Appropriate pulse energy
is used to achieve full depth of penetration typically up to 3 mm
thickness of Nb or Nb alloyed components used.
The full penetration weld (keyhole welding) is obtained from inside
surface of the wall of the cavity with the help of a Type `A`
nozzle (NA), which has three concentric tubes. The laser beam
travels through the innermost tube. The outermost and innermost
tube enclosures are used for supplying high velocity inert gas to
flow out to the weld region and the middle tube/enclosure is used
for sucking out the evaporated material due to laser material
interaction. The welding parameters are such that pulse profile is
varied in time phase to minimize energy deposited for same
penetration depth thereby reducing distortion. The welding nozzle
is inclined at an angle of 15.degree.-45.degree. and more
preferably 30.degree. to vertical to achieve good surface finish
for same depth of penetration.
In the second step the inclined nozzles (NB) are brought at the
welded joint. The beams are inclined and the weld plume rises
perpendicular to the surface. The evaporated material formed
because of laser material interaction is sucked out through an
evacuation enclosure/suction device (SCD), which is seen in the
plan view only. Similarly, the two nozzles (NB) are visible in the
top view only. In this operation a very smooth bead is
obtained.
As a next step, said inside surface of the wall of welded joint is
viewed with the help of a viewing mechanism that uses the same lens
mechanism, which will be used to weld the joint. The surroundings
are viewed with the help of a boroscope. In case there is a
requirement then the weld is repaired/re welded by suitable nozzle
NA or NB with sucking out of evaporated material as has been done
throughout the process. If the need be then a low energy defocused
beam is used to smoothen the undulations on the surface of weld
bead after laser cleaning. FIG. 1A depicts an enlarged illustration
of the arrangement of half cells forming the dumbbells housed in
the Type I rig of FIG. 1 and a section AA of the same enlarged view
in the dumbbell;
A separate optics (OP) is provided at a suitable location on the
top cover flange (CF) of the vacuum vessel (W) for viewing the fit
up and process from outside the chamber. FIG. 1 shows only one of
these two beams in front view. The joint once made is viewed to see
if there is some area left unwelded, repair is performed at the
same place in the next pass. A retractable mechanism provided on a
central shaft at the axial location allow approach of the vertical
single nozzle (NA) and the paired inclined nozzles (NB), one after
the other in sequence, to access the weld area, movements of which
can be controlled from outside of the rig.
The chamber is then opened and stiffening rings (SR) over the iris
joints of the SCRF cavity are fitted with suitable fixtures. The
vessel is now closed and evacuated to a vacuum of 10.sup.-5 mbar.
Suitable ultra pure inert gas-either of helium, or argon is then
purged in this vessel to displace the air trapped inside. After a
couple of such cycles, it can be reasonably ensured that no
contaminating gas is present in the chamber. The stiffening ring
(SR) joint, which is in the RF field free region, is welded by
keyhole welding technique. The joint is viewed with the help of a
viewing mechanism that uses the same lens mechanism, which was used
to weld the joint. If there is some region, which is unwelded,
then, it is welded again without opening the chamber.
The dumbbell (DB) assembly is left for some time to come to ambient
temperature. As the pulsed energy input is selectively controlled,
there is no significant rise in temperature and hence distortion
and shrinkage is controlled. The gas, which is being provided, also
helps in cooling. The dumbbell (DB) assembly is taken out and
thoroughly inspected for any region, which has less smoothness in
bead and/or some region, which has certain areas, which need to be
cleaned. The dumbbell is again fitted inside the chamber and
manipulating mechanism is used to defocus the beam. The vessel is
now closed and evacuated to a vacuum of 10.sup.-5 mbar. Suitable
ultra pure inert gas-either of helium, or argon is then purged in
this vessel to displace the air trapped inside. After a couple of
such cycles, it can be reasonably ensured that no contaminating gas
is present in the chamber. The cleaning of the weld region is
performed with very short pulses of low energy and then smoothening
of the bead is performed with defocused low energy pulses.
Reference is now invited to the accompanying FIG. 1 that
illustrates the schematic arrangement of the welding rig of Type I
having a simple cylindrical vacuum chamber with motorized holder
(MH)/attachments for holding and rotating the dumbbells (DB) by
rotary mechanism for welding of stiffening rings from outside
fitted on the dumbbells circumferentially covering the iris joint
of SCRF cavity by suitably locating and focusing the laser beam
using the weld nozzle of type `A` at the exact weld location by
means of nozzle alignment mechanism (NAM) and feed through (FT).
The entire assembly and the welding can be viewed from out side
through a suitable optics (OP) on the top cover flange (CF) fixed
on the vacuum vessel (VV) which is demountable. The accompanying
FIG. 1B also shows the laser welding nozzle position for
single/dual laser beam welding of Iris joint from inside surface of
the wall of the cavity dumbbell, as already described.
STEP-2: Fabricating the End Group by Welding the Joints by Laser
Welding Inside the Type I Welding Rig.
The main body (MB) of the end group (EG) is first provided as shown
in FIG. 4a. This is obtained by machining it out of a single block
of niobium. Wire cut EDM process is used to remove a cylindrical
block of niobium and this block is used to make HOM coupler and
other flanges to be used with end group. Machining process
performed on a turn mill centre make the other features of the end
group main body.
The end group (EG) joints are usually welded using Type I welding
rig.
Reference is now invited to the accompanying FIG. 4c that shows how
to weld the end group components one by one. The basic methodology
is same as illustrated in Step 1. The fixtures and set-up will only
differ. The same Type I rig is used to carry out all such welding
of the end group components.
Reference is also invited to the accompanying FIG. 4b, which shows
the other components of end group comprising the HOM coupler, the
NbTi ring and the coupler flange used in end group
assembly/fabrication.
Reference is now invited to the accompanying FIG. 4c, which shows
welding of NbTi ring to the end group flange. It is clearly
apparent from the accompanying FIG. 4(b) that the nozzle is
vertically positioned on the weld location for welding the NbTi
ring fitted at top of the end group body such that complete
penetration is achieved by keyhole welding from outside.
Reference is now invited to the accompanying FIG. 4d, which shows
welding of HOM coupler flange to main body of end group. Here,
keyhole welding and conduction welding/smoothening is achieved with
only one beam through a nozzle which can rotate about it's own
axis.
Reference is now invited to the accompanying FIG. 4e, which shows
welding of half-cell to main body of end group. It is the welding
in RF field region taking place from inside surface of the wall of
the SCRF cavity. Care is taken to avoid any surface irregularity
due to welding at this location and accordingly the weld parameters
are selectively used while ensuring full penetration weld. A finish
welding using dual laser beam from inside surface of the wall of
this joint serves the purpose of meeting the requirement of RF
field region joint.
Methodology for Welding End Group Components
The components are assembled with the main body with the help of
fixtures one by one. The joints will be made in different settings.
The joint is then checked for proper fit up visually from both
inside and outside. The manipulating mechanism is used to ensure
that the beam focal point is perfectly matched with butting line.
As the next step a He--Ne laser is then used to check the focal
point of the impending beam from inside. Similarly He--Ne laser is
then used to check the focal point of the impending beam/beams from
outside for joints in RF field free region.
The vessel is now closed and evacuated to a vacuum of 10.sup.-5
mbar. Suitable ultra pure inert gas--either of helium, or argon is
then purged in this vessel to displace the air trapped inside.
After a couple of such cycles, it can be reasonably ensured that no
contaminating gas is present in the chamber.
The joints, which are in the RF field free region, are welded by
keyhole welding technique. The joint is viewed with the help of a
viewing mechanism that uses the same lens mechanism, which was used
to weld the joint. If there is some region, which is unwelded, then
it is welded again, without opening the chamber.
The joints, which are in the RF field region, are first welded by
keyhole welding technique followed by conduction welding technique.
The joint is viewed with the help of a viewing mechanism that uses
the same lens mechanism, which was used to weld the joint. If there
is some region, which is unwelded, then it is welded again without
opening the chamber. The cleaning of the weld region is performed
with very short pulses of low energy followed by smoothening of the
bead, which is performed with low energy pulses.
STEP-3: Welding Equator Joints with Help of Insert in the Type II
Welding Rig, to Form Cavity with the Pre-Welded Dumbbell
Assemblies.
Reference, is first invited to the accompanying FIG. 5 to FIG. 7
that illustrates laser welding of equator joint of cavity using
Type II welding Rig and the Insert assembly for precise fit up and
online distortion monitoring and control of such weld joint. Such a
rig is designed to weld the joints of the SCRF cavity falling
inside the RF field region. Such a rig essentially consists of a
vacuum vessel made of stainless steel (SS 316L). The ends of the
vessel are such that it can be opened from one side. There are a
few windows on the lateral surface for viewing. The weld rig vessel
has provision to be attached to a vacuum pump. The tube carrying
the optical fiber also carries inert gas. This tube is introduced
into the weld chamber with the help of feed through. The same
optical fiber is used to couple four types of pulses in
time-sharing basis. A very special feature of this welding rig is
the online distortion monitoring by providing strain gauges (SG)
between cells. The first part of this welding rig is an insert (IN)
as illustrated in the accompanying FIG. 5, which is made by using
two circular flanges (CFS) that are held together by three tie rods
(TR) circumferentially spaced 120.degree. apart. It is clearly
apparent from FIG. 5, that the half-cells/dumbbells (DB) have been
assembled together with precisely contoured spacers (SP). All the
dumbbells made of half-cells are precisely machined at edges and
assembled together, with strain gauges (SG) mounted in between two
consecutive half cells/dumbbells (DB) to monitor and control
distortion/shrinkage during welding. The half-cells, which were
welded to make a dumbbell (DB) in Type I welding rig, are precisely
assembled in this insert (IN). The precisely shaped spacers (SP)
are used for exact location maintaining co-axiality of the
dumbbells to form a complete cavity. Strain gauges (SG) are also
assembled in this insert to monitor the distortions online during
welding.
This whole assembly is then slid inside the vacuum vessel as shown
in the accompanying FIG. 6. Now welding of equator joint completely
from inside surface of the wall of cavity is taken up. First it is
welded with a high energy pulse which has a particular variation of
energy in time phase and then in the next step smoothening the weld
bead with the help of a dual beam welding technique, as already
illustrated with reference to welding of iris joints in type I Rig
is done.
The accompanying FIG. 6 shows the schematic illustration of the
welding rig of type II along with insert carrying the dumbbells for
welding. According to the present invention this rig comprises of a
vacuum vessel with different devices/attachments, to carry out
laser welding of joints of Niobium components of SCRF cavity,
specifically for the equator joints of SCRF cavity.
The welding methodology follows the sequence, viewing butting
edges--keyhole welding--viewing--repairing--viewing--dual beam
conduction welding/smoothening--viewing the entire region with
boroscope--laser cleaning--final smoothening with defocused
laser--final inspection with boroscope.
The welding operation is performed in two steps in this rig. In the
first step, keyhole welding process is used with the help of a
beam/nozzle which is inclined at an angle in the range of
15.degree. to 45.degree. and preferably 30.degree. to the vertical
in the plane of welding. The energy is varied in time phase in this
case for better penetration. The welding nozzle used is Type `A`
having three concentric tube type annular passages to carry laser
beam as well as the desired flow rate of protective gas and suction
device to remove evaporated plasma material generated during
welding. Here, the lens assembly does not move and it remains
within the hollow cylindrical shaft. The three concentric tube type
enclosures move up and down. The outermost and innermost tube
enclosures are used for high velocity inert gas to flow out whereas
the middle enclosure is used for sucking out the evaporated
material due to laser material interaction.
The `A` type nozzle that is used for keyhole welding is shown in
FIG. 2, illustrating the detailed constructional features in a
sectional view. The full penetration weld (keyhole welding) will be
obtained from inside surface of the wall of cavity with the help of
this nozzle as shown which has three concentric tubes/annular
passages. The laser beam is delivered through the innermost tube.
The outermost and innermost tube supplies gas at high velocity and
the middle tubels connected to suction device. The nozzle has the
flexibility to access remote weld locations. Its different elements
are adapted to carry out the combined tasks of laser welding,
providing two gas jets and suction mechanism to remove the metal
vapors or other evaporated material from the weld joint location.
The lens assembly remains stationary and the enclosures move up and
down.
In the second stage conduction welding operation is performed in
order to obtain a very smooth finish after keyhole welding
operation is over. Dual beam welding technique is used for this. A
major advantage of dual laser beam at inclination to vertical
orientation from two sides to the weld spot is that the evaporated
material generated due to laser material interaction rises up
perpendicular to the surface of the joint and is easily collected
by an enclosure which is adapted to move in up and down
directions.
The equator welding is performed in the SS chamber shown in FIG. 5.
The insert is assembled inside the chamber, joints are then checked
for proper fit up visually from inside. The manipulating mechanism
is used to ensure that the beam focal point is perfectly matched
with half-cell butting line. As the next step a He--Ne laser is
then used to check the focal point of the impending beam from
outside. The chamber is now closed and is evacuated to a vacuum of
10.sup.-5 mbar. Suitable ultra pure inert gas-either of helium, or
argon is then purged in this vessel to displace the air trapped
inside. After a couple of such cycles, it can be reasonably ensured
that no contaminating gas is present in the chamber.
The equator joints, which are in the RF field region, are welded by
keyhole welding technique. The use of varying energy in time within
a pulse and pulse overlap in space are used to generate low heat
affected zone and full depth of penetration. The pulse to be used
is depicted in FIG. 11. The welding parameters are such that pulse
profile is varied in time phase to minimize distortion but still
obtain full depth penetration. The temporal profile of the pulse is
tailored for Niobium or its alloys preferably by varying, the
energy within the pulse in time domain in a near trapezoidal shape
such that initial preheating of surface results in higher
penetration and the tapering off the energy near the end is done in
such a way to avoid micro cracks/porosity/other defects, whereby
the overall pulse shape ensures high penetration depth and low
distortion.
The joint is viewed with the help of a viewing mechanism that uses
the same lens mechanism, which was used to weld the joint. If there
is some region left unwelded, then it is welded again without
opening the chamber. The insert is taken out and spacers and strain
gauges are uninstalled.
FIG. 7 Shows the mechanism, which is used in Type II welding rig.
It illustrates some important features of this design. There is no
ball screw which goes inside the welding chamber as we want to
utilize the little space available inside (which is the case with
most of the elliptical cavities), for different welding
contraptions like nozzles and optical fibers. The welding rig uses
encoders and other accessories so that precise position of the
nozzles can be known from outside. Boroscopes (BS) are provided to
inspect fit up of a joint prior to start of weld and also the weld
joint and surrounding regions after welding is over for any
possible defect requiring rectification.
The methodology will be same i.e. evacuation, purging with pure
gas, welding with sucking out of evaporated material and the
allowing the assembly to cool before it is taken out. Initial data
from strain gauges is generated through a few trial runs for a few
fit ups of dumbbells of SC cavities and a welding sequence is
standardized.
A specially designed insert is used to hold the dumbbell assembly
by three tie rods circumferentially spaced 120.degree. apart. This
whole assembly is then slid inside the vacuum vessel of Type II
weld rig as shown in the accompanying FIG. 7.
All the joints are made in single setting. The joint is first
viewed through optical fiber and lens mechanism to ensure that the
beam focal point is perfectly matched with butting line for this
task a He--Ne laser (or any suitable laser in visible spectrum) is
used to check the focal point of the impending beam while viewing
from outside. The vessel is now closed and evacuated to a vacuum of
10.sup.-5 Mbar. Suitable ultra pure inert gas--either helium, or
argon is then purged in this vessel to displace the air trapped
inside. After a couple of such cycles, it can be reasonably ensured
that no contaminating gas is present in the chamber.
The equator joints, which are in the RF field region, are welded
first by keyhole welding technique as illustrated in FIG. 8, in
`section B-B` of FIG. 7. Here the lens assembly does not move and
it remains within the hollow cylindrical shaft. The three
concentric tube type enclosures move up and down. The outermost and
innermost concentric annular enclosures are used for high velocity
inert gas to flow out whereas the middle enclosure is used for
sucking out the evaporated material due to laser material
interaction. The nozzle is inclined in the plane of welding as is
evident from the figure. The preferred angle is 30.degree. for
keyhole welding using single laser beam.
The joint is viewed online with the help of a viewing mechanism
that uses the same lens mechanism, which was used to weld the
joint. If there is some region, which is left unwelded then it is
welded again without opening the chamber.
The smoothening of the bead is performed in the second step with
low energy pulses using two inclined laser beams delivered and
directed by a separate set of nozzles. A retractable evaporating
material suction arrangement has been provided with its open end
close to the weld seam having means for collection through
concentric enclosure. This is illustrated in the accompanying FIG.
9, as indicated in section `A-A` of FIG. 6. This operation is
performed in order to obtain a very smooth finish after
keyhole-welding operation is over.
Dual beam welding technique is used here. A major advantage here is
that the evaporated material as a result of laser material
interaction rises up perpendicular to the surface of the joint and
is collected by an enclosure, which can be moved up and down. The
lens assembly has a focal length such that the two beams are
focused on the butting edge.
Accompanying FIG. 3 shows the enlarged view of the deployment of
two inclined nozzles to illustrate the principle of dual beam
welding with removal of evaporated material. It shows the welding
nozzle of Type B (NB), arranged in welding rig of type II (WR-II)
according to the present invention. The nozzle set up alms to carry
out laser welding of joints of Niobium components of SCRF cavity,
which are in the RF field region. The welding is performed from
Inside surface of the wall of cavity with conduction welding
technique after keyhole welding is over. The main feature is the
enclosure for collecting the evaporated material, which can move up
and down telescopically. A typical case of welding with the help of
two beams, which make an angle of 45.degree. with the vertical has
been illustrated.
In the next step, without opening the chamber, the boroscope fitted
with the central shaft is used to view the entire weld region as
well as the surroundings. The boroscopic inspection set-up and
procedure has been schematically illustrated in the accompanying
FIG. 10. In the last step the assemblage of dumbbells is now
withdrawn from the vessel.
It is thus possible by way of the present invention to develop
superconducting radio frequency cavity fabricated by laser welding
of Niobium and its alloy based cavities such that said laser
welding would enable achieving Niobium or it's alloys based
superconducting radio frequency (SCRF) cavities, comprising of at
least one component made of Niobium or it's alloys which are joined
by laser welding from inside surface of the wall of the cavity with
depth of penetration of welding ranging from greater than half the
thickness to full depth of the material being welded. This in turn
would facilitate the production of much required weld joints of the
cavity with good surface finish, strength and mitigation of risks
of field emission so that consistent performance from the RF joints
can be obtained while in service for charged particle acceleration
in these cavities. Importantly, the system, apparatus and method of
the invention is specially configured to carry out the full depth
of penetration of welding Nb or Nb alloyed components of thickness
over 1 mm and typically 3 mm thickness or more from inside surface
of the wall of the SCRF cavity using welding rigs of Type I and
Type II and special nozzle configuration/orientations in two
steps--firstly keyhole welding using high energy laser pulse and
then in second step weld smoothening by low energy laser pulse with
dual laser beam selectively directed inclined to vertical.
Retractable concentric tubular evacuation arrangement is provided
to suck away the evaporated materials from the laser material
interaction, while the lens and focusing means is housed inside the
hollow axial central core of shaft/cable of the Type II rig. Also
laser weld is carried out precisely at different weld locations for
various components of the SCRF cavity both in RF field region and
RF field free region with minimum human intervention, minimum
distortion/rejection, superior and consistent weld quality with
narrow HAZ, low distortion and shrinkage, high strength, weld
surface free of any sharp point and faster production of such
cavities, thus enabling fabrication of such SCRF cavity of superior
quality and reliable performance for accelerating charged particles
without the risks of any filed emission during operation of the
cavity.
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